Stage system, lithographic apparatus, method for positioning and device manufacturing method

ABSTRACT

A system for positioning, a stage system, a lithographic apparatus, a method for positioning and a method for manufacturing a device in which use is made of a stage system. The stage system has a plurality of air bearing devices. Each air bearing device has: a gas bearing body which has a free surface, a primary channel which extends through the bearing body and has an inlet opening in the free surface, and a secondary channel system which extends through the bearing body and which has a plurality of discharge openings in the free surface. The flow resistance in the secondary channel system can be higher than the flow resistance in the primary channel.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase entry of PCT patentapplication no. PCT/EP2017/064265, which was filed on Jun. 12, 2017,which claims the benefit of priority of European patent application no.16177447.6, which was filed on Jul. 1, 2016, and European patentapplication no. 17158730.6, which was filed on Mar. 1, 2017, and whichare incorporated herein in their entirety by reference.

BACKGROUND Field of the Invention

The present invention relates to a stage system, a lithographicapparatus, a method for positioning and a method for manufacturing adevice in which use is made of a stage system.

Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In such a case, a patterning device, which isalternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Conventional lithographicapparatus include so-called steppers, in which each target portion isirradiated by exposing an entire pattern onto the target portion atonce, and so-called scanners, in which each target portion is irradiatedby scanning the pattern through a radiation beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

A lithographic apparatus often comprises a stage system for positioningthe substrate and/or the patterning device. The substrate and thepatterning device need to be positioned very accurately. The positioningcan be adversely affected when the substrate or patterning device is notentirely flat, but for example warped, concave, convex, or havinganother low order Zernike shape.

WO2008/156366 proposes to flatten a substantially planar object prior toclamping it on an object table by using jets of pressurised gas. Theobject table is provided with a recess which is connected to a vacuumsource, and with nozzles that are adapted to emit jets of pressurizedgas. The balance between the attracting force of the vacuum and therepelling force caused by the jets makes that the object floats at adistance above the object table, and that deviations from the flat shapeof the object are corrected to some extent.

The known system is not very stable.

SUMMARY

It is desirable to provide a stable system and method which allowsaccurate positioning of an object relative to a surface.

According to an embodiment of the invention, a stage system is providedfor positioning an object, which stage system comprises:

-   an object table adapted to support the object to be positioned,-   which object table is provided with a plurality of air bearing    devices,-   wherein each air bearing device comprises:    -   an air bearing body, which has a free surface,    -   a primary channel which extends through the air bearing body and        has an inlet opening in the free surface,    -   a secondary channel system which extends through the air bearing        body and which has a discharge opening in the free surface,-   wherein the flow resistance in the secondary channel system is    higher than the flow resistance in the primary channel.

In another embodiment of the invention, there is provided a lithographicapparatus arranged to transfer a pattern from a patterning device onto asubstrate, wherein the lithographic apparatus comprises a stage systemaccording to the invention.

In another embodiment of the invention, there is provided a lithographicapparatus comprising:

-   -   an illumination system configured to condition a radiation beam;    -   a support constructed to support a patterning device, the        patterning device being capable of imparting the radiation beam        with a pattern in its cross-section to form a patterned        radiation beam;    -   a substrate table constructed to hold a substrate; and    -   a projection system configured to project the patterned        radiation beam onto a target portion of the substrate,

-   wherein the substrate table is provided with a plurality of air    bearing devices,

-   wherein each air bearing device comprises:    -   an air bearing body, which has a free surface,    -   a primary channel which extends through the air bearing body and        has an inlet opening in the free surface,    -   a secondary channel system which extends through the air bearing        body and which has a discharge opening in the free surface,

-   wherein the flow resistance in the secondary channel system is    higher than the flow resistance in the primary channel.

In another embodiment of the invention, there is provided a devicemanufacturing method comprising transferring a pattern from a patterningdevice onto a substrate, wherein use is made of a stage system accordingto the invention.

In another embodiment of the invention, there is provided a method forpositioning an object, which method comprises the following steps:

-   -   arranging an object on or above the object table of a stage        system according to the invention,    -   making pressurized gas flow out of the discharge openings of the        secondary channel systems of at least one air bearing device of        the stage system according to the invention while simultaneously        applying a sub-atmospheric pressure to the inlet of the primary        channel of at least one air bearing device of the stage system        according to the invention, thereby keeping the object in a        position spaced apart from the object table in a direction        perpendicular to the object table.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 schematically shows a first embodiment of a system according tothe invention,

FIG. 3 schematically shows a further embodiment of the system accordingto the invention,

FIG. 4 schematically shows a further embodiment of the system accordingto the invention,

FIG. 5 schematically shows a side view of an example of an object tableof stage system of a lithographic apparatus,

FIG. 6 schematically shows an example of an air bearing device as can beused in the embodiment of FIG. 5 ,

FIG. 7 schematically shows a variant of a stage system according to theinvention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus includes an illuminationsystem (illuminator) IL configured to condition a radiation beam B (e.g.UV radiation or any other suitable radiation), a mask support structure(e.g. a mask table) MT constructed to support a patterning device (e.g.a mask) MA and connected to a first positioning device PM configured toaccurately position the patterning device in accordance with certainparameters. The apparatus also includes a substrate table (e.g. a wafertable) WT or “substrate support” constructed to hold a substrate (e.g. aresist coated wafer) W and connected to a second positioning device PWconfigured to accurately position the substrate in accordance withcertain parameters. The apparatus further includes a projection system(e.g. a refractive projection lens system) PS configured to project apattern imparted to the radiation beam B by patterning device MA onto atarget portion C (e.g. including one or more dies) of the substrate W.

The illumination system IL may include various types of opticalcomponents, such as refractive, reflective, magnetic, electromagnetic,electrostatic or other types of optical components, or any combinationthereof, for directing, shaping, or controlling radiation.

The support structure MT supports, i.e. bears the weight of, thepatterning device MA. It holds the patterning device MA in a manner thatdepends on the orientation of the patterning device MA, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The mask support structure can use mechanical, vacuum, electrostatic orother clamping techniques to hold the patterning device. The masksupport structure may be a frame or a table, for example, which may befixed or movable as required. The mask support structure may ensure thatthe patterning device is at a desired position, for example with respectto the projection system. Any use of the terms “reticle” or “mask”herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section so as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device MA may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “radiation beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables or “substrate supports” (and/or two or more masktables or “mask supports”). In such “multiple stage” machines theadditional tables or supports may be used in parallel, or preparatorysteps may be carried out on one or more tables or supports while one ormore other tables or supports are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques can beused to increase the numerical aperture of projection systems. The term“immersion” as used herein does not mean that a structure, such as asubstrate, must be submerged in liquid, but rather only means that aliquid is located between the projection system and the substrate duringexposure.

Referring to FIG. 1 , the illuminator IL receives a radiation beam froma radiation source SO. The radiation source SO and the lithographicapparatus may be separate entities, for example when the source is anexcimer laser. In such cases, the radiation source SO is not consideredto form part of the lithographic apparatus and the radiation beam ispassed from the radiation source SO to the illuminator IL with the aidof a beam delivery system BD including, for example, suitable directingmirrors and/or a beam expander. In other cases the radiation source SOmay be an integral part of the lithographic apparatus, for example whenthe source is a mercury lamp. The radiation source SO and theilluminator IL, together with the beam delivery system BD if required,may be referred to as a radiation system.

The illuminator IL may include an adjuster AD configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asG-outer and G-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may include various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross section.

The radiation beam B is incident on the patterning device MA, which isheld on the support structure MT, and is patterned by the patterningdevice. Having traversed the patterning device MA, the radiation beam Bpasses through the projection system PS, which focuses the beam onto atarget portion C of the substrate W. With the aid of the secondpositioning device PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioningdevice PM and another position sensor (which is not explicitly depictedin FIG. 1 ) can be used to accurately position the patterning device MAwith respect to the path of the radiation beam B, e.g. after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe support structure MT may be realized with the aid of a long-strokemodule (coarse positioning) and a short-stroke module (finepositioning), which form part of the first positioning device PM.Similarly, movement of the substrate table WT or “substrate support” maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using mask alignment marks M1, M2 andsubstrate alignment marks P1, P2. Although the substrate alignment marksP1, P2 as illustrated occupy dedicated target portions, they may belocated in spaces between target portions C (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the mask alignmentmarks M1, M2 may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

-   1. In step mode, the support structure MT and the substrate table WT    are kept essentially stationary, while an entire pattern imparted to    the radiation beam is projected onto a target portion C at one time    (i.e. a single static exposure). The substrate table WT is then    shifted in the X and/or Y direction so that a different target    portion C can be exposed. In step mode, the maximum size of the    exposure field limits the size of the target portion C imaged in a    single static exposure.-   2. In scan mode, the support structure MT and the substrate table WT    are scanned synchronously while a pattern imparted to the radiation    beam is projected onto a target portion C (i.e. a single dynamic    exposure). The velocity and direction of the substrate table WT or    “substrate support” relative to the support structure MT may be    determined by the (de-)magnification and image reversal    characteristics of the projection system PS. In scan mode, the    maximum size of the exposure field limits the width (in the    non-scanning direction) of the target portion in a single dynamic    exposure, whereas the length of the scanning motion determines the    height (in the scanning direction) of the target portion.-   3. In another mode, the support structure MT is kept essentially    stationary holding a programmable patterning device, and the    substrate table WT is moved or scanned while a pattern imparted to    the radiation beam is projected onto a target portion C. In this    mode, generally a pulsed radiation source is employed and the    programmable patterning device is updated as required after each    movement of the substrate table WT or in between successive    radiation pulses during a scan. This mode of operation can be    readily applied to maskless lithography that utilizes programmable    patterning device, such as a programmable mirror array of a type as    referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 shows a first embodiment of a system 1 according to theinvention. The system 1 can for example be a stage system.

The system 1 as shown in FIG. 2 comprises member 10, which has a surface11. In this example, the surface 11 is the top surface of member 10. Themember 10 may for example be an object table of a stage system.

The member 10 is provided with a plurality of air bearing devices 20.Each air bearing device 20 comprises an air bearing body 21, which has afree surface 22. The free surface 22 is not covered by other physicalcomponents of the system. In this embodiment, it forms part of the outersurface of the member 10. In use, and in particular after an optionalclamping the object on the member 10, the free surface 22 may be coveredby the object that is supported on the member 10. In the example of FIG.2 , the free surfaces 22 of the air bearing bodies 21 are arranged flushwith the surface 11 of the member 10, but this is not necessary. In FIG.2 , four air bearing devices are provided, but alternatively any othernumber of air bearing devices may be provided, in particular two ormore.

Each air bearing device 20 further comprises a primary channel 23 whichextends through the air bearing body 21. The primary channel 23 has aninlet opening 24 in the free surface 22.

Each air bearing device 20 further comprises a secondary channel system25 which extends through the air bearing body 21. The secondary channelsystem 25 has a plurality of discharge openings 26 in the free surface22 of the air bearing body 21. For reasons of clarity, only a fewdischarge openings are provided with a reference numeral in FIG. 2 . Thesecondary channel system 25 may have only one discharge opening 26 inthe free surface 22 of the air bearing body 21.

In the example shown in FIG. 2 , the air bearing body 21 comprises aporous material having interconnected cavities. The secondary channelsystem 25 is formed by the interconnected cavities in the porousmaterial. The porous material is for example a sintered ceramicmaterial, e.g. SiSiC. Optionally, the faces of the porous materialthrough which no pressurised gas should escape are sealed.

In an alternative embodiment, not shown in the drawing, the secondarychannel system 25 in the air bearing body may be formed by channels thatare for example machined, e.g. drilled, or etched into the air bearingbody.

Preferably, the primary channel 23 is connectable to a source ofsub-atmospheric pressure, and the secondary channel system 25 isconnectable to a source of pressurised gas. In use, when an object isarranged on or above the surface 11, a pressurised gas can be suppliedbetween the object and the surface 11 and/or between the object and thefree surfaces 22, via the discharge openings 26 of the secondary channelsystem 25. At the same time, a sub-atmospheric pressure is provided atthe inlet 24 of the primary channel 23. The pressurised gas pushes theobject away from the surface 11, while the sub-atmospheric pressurepulls the object towards surface 11. The balance between the pushingforce that the pressurised gas exerts on the object and the pullingforce that the sub-atmospheric pressure exerts on the object allows toposition the object in z-direction relative to the surface 11, whichsurface 11 extends in the x-y plane.

The flow resistance in the secondary channel 25 system is higher thanthe flow resistance in the primary channel 23. This stabilizes thesystem, because the higher flow resistance prevents that a force inz-direction, for example caused by tilting of the object around thex-axis or y-axis, forces the pressurised air back into the secondarychannel system 25. If pressurised gas would be forced back into thesecondary channel system, the balance between the pushing force and thepulling force would be disturbed (at least temporarily), making thesystem unstable. It even may result in that the object hits the surface11 before the balance between the pushing force and the pulling force isre-established. This is called “hammering”, and is not desired. Inaddition, the feature that the flow resistance in the secondary channelsystem 25 is higher than the flow resistance in the primary channel 23results in a high tilt stiffness of the system. Preferably, the flowresistance in the secondary channel 25 system is also higher than theflow resistance in the gap between the free surface 22 and the object tobe positioned.

So, the flow resistance in the secondary channel system 25 being higherthan the flow resistance in the primary channel 23 increases thestability and the reliability of the system.

Optionally, the member 10 may comprise channels 12 to connect theprimary channel 23 of the air bearing device 20 to a source ofsub-atmospheric pressure and further channels 13 to connect thesecondary channel system 25 of the air bearing device 20 to a source ofpressurised gas.

In the example of FIG. 2 , in the air bearing devices 20 the dischargeopenings 26 of the secondary channel system 25 surround the inletopening 24 of the primary channel 23. This provides further stability tothe system, because the pushing force exerted by the pressurised gas isdistributed over a relatively large area, often evenly distributed overthis area. In addition, pushing forces on opposite sides of the inletopening 24 of the primary channel 23 may counteract each other, whichalso further stabilises the system. This results in a high tiltstiffness of the system. This effect is even more substantial when aporous material is used for the air bearing body, because of the largenumber of discharge openings 26 in such an embodiment.

In a variant of the embodiment of FIG. 2 , the system further comprisesan sub-atmospheric pressure source which is in fluid communication withthe primary channel 23 of at least one air bearing device 20.Alternatively or in addition, in this variant, the system furthercomprises a source of pressurised gas which is in fluid communicationwith the secondary channel system 25 of at least one air bearing device20.

FIG. 3 schematically shows a further embodiment of the system accordingto the invention.

In the embodiment of FIG. 3 , the member 10 is provided with atwo-dimensional array of air bearing devices 20. In this example, allair bearing devices 20 are identical, but this is not necessary. The airbearing devices 20 in the embodiment of FIG. 3 are for example the sameas the air bearing devices 20 as shown in FIG. 2 or as shown in FIG. 6 .Again, the member 10 may for example be an object table of a stagesystem or member comprising a guide surface.

In the embodiment of FIG. 3 , the air bearing devices 20 are arranged ina rectangular grid. They are distributed over the surface 11 of themember 10, so they can act locally on an object that is present on orabove the member 10. This allows not only to position the object, butalso to change the shape of the object. For example, it is possible tocorrect warp, concavity, convexity, or another low order Zernike shape,or local deformation or other deviations from the flat shape of theobject.

In the embodiment of FIG. 3 , an air bearing device 20* is provided atthe centre of the member 10. This further increases the stability of thesystem, as the actions of air bearing devices 20 on opposite sides ofthe centre of the member 10 may counteract each other.

FIG. 4 schematically shows a further embodiment of the system accordingto the invention.

In the embodiment of FIG. 4 , the member 10 is provided with an array ofair bearing devices 20. In this example, all air bearing devices 20 areidentical, but this is not necessary. The air bearing devices 20 in theembodiment of FIG. 4 are for example the same as the air bearing devices20 as shown in FIG. 2 or as shown in FIG. 6 . Again, the member 10 mayfor example be an object table of a stage system or member comprising aguide surface.

In the embodiment of FIG. 4 , the air bearing devices 20 are arranged ina polar grid. They are arranged on lines meeting in the centre of thepolar grid. In the example of FIG. 4 , the air bearing devices arearranged in concentric circles around the centre of the polar grid. So,a number of air bearing devices 20 has the same distance to the centreof the polar grid.

The air bearing devices are distributed over the surface 11 of themember 10, so they can act locally on an object that is present on orabove the member 10. This allows not only to position the object, butalso to change the shape of the object. For example, it is possible tocorrect warp, concavity, convexity, or another low order Zernike shape,local deformation or other deviations from the flat shape of the object.

In the embodiment of FIG. 4 , an air bearing device 20* is provided atthe centre of the member 10. This further increases the stability of thesystem, as the actions of air bearing devices 20 on opposite sides ofthe centre of the member 10 may counteract each other.

In FIGS. 2, 3 and 4 , the air bearing bodies 21 have a rectangular freesurface 22, and are generally box-shaped. Alternatively, other shapesare possible, for example cylindrical air bearing bodies, e.g. with acircular or elliptical free surface.

Optionally, in any of the embodiments of the FIGS. 2, 3 and 4 , theplurality of air bearing devices 20 is adapted to change the shape of anobject which is to be arranged on the member 10 by controlling thepressure and/or gas flow rate at the inlet opening 24 of a primarychannel 23 and/or the pressure and/or gas flow rate at the dischargeopenings 26 of the secondary channel system 25 of at least one airbearing device 20.

Optionally, the control of the pressure and/or gas flow rate at theinlet 24 of a primary channel 23 and/or the pressure and/or gas flowrate at the discharge openings 26 of a secondary channel system 25 of atleast one air bearing device 20 is based on the expected and/or measuredshape of the object.

In a variant, any of the embodiments of the FIGS. 2, 3 and/or 4 furthercomprises a measurement system which is adapted to obtain shape data ofan object arranged on or above the member 10 during activation of theair bearing devices 20. The measurement system optionally comprisescapacitive sensors, and/or inductive sensors and/or an interferometer.Optionally, one or more parts of the measurement system are arranged onor in the member 10, so that measurement of the object can take placewhile the object is arranged at or above the member 10. Alternatively orin addition, the measurement system may be arranged such thatmeasurement of the object takes place before the object is arranged ator above the member 10.

In this variant, the embodiment further comprises a control device forcontrolling the pressure and/or gas flow rate at the inlet opening 24 ofa primary channel 23 and/or the pressure and/or gas flow rate at thedischarge openings 26 of a secondary channel system 25 of at least oneair bearing device 20. The control device is adapted to receive theshape data from the measurement system and to control the pressureand/or gas flow rate at the inlet opening 24 of the primary channel 23and/or the pressure and/or gas flow rate at the discharge openings 26 ofthe secondary channel system 25 of the at least one air bearing device20, based on the received shape data.

Optionally, in any of the embodiments of the FIGS. 2, 3 and/or 4 , theplurality of air bearing devices comprises multiple air bearing groups.Each air bearing group comprises at least one air bearing device 20. Forexample, in the embodiment of FIG. 3 , each row of air bearing devices20 extending in the x-direction may form an air bearing group, or eachrow of air bearing devices 20 extending in the y-direction may form anair bearing group. In the embodiment shown in FIG. 4 , for example theair bearing devices 20 that have the same distance to the centre of thepolar grid (so the air bearing devices on the same circle) may form anair bearing group.

Within an air bearing group, the pressure and/or gas flow rate at theinlets of the primary channels is in this embodiment controllableseparately from the pressure and/or gas flow rate at the inlet openingof the primary channels in another air bearing group. This allows tocontrol the local forces that are exerted on the object, which allows tochange the shape of the object. Optionally, the pressure and/or gas flowrate at the inlet opening of the primary channels is controllableindependently from the pressure and/or gas flow rate at the inletopenings of the primary channels in another air bearing group.

Alternatively or in addition, the pressure and/or the gas flow rate atthe discharge openings of the secondary channel systems in at least oneair bearing group is controllable separately from the pressure and/orgas flow rate at the discharge openings of secondary channel systems inanother air bearing group. This allows to control the local forces thatare exerted on the object, which allows to change the shape of theobject. Optionally, the pressure and/or the gas flow rate at thedischarge openings of the secondary channel systems in at least one airbearing group is controllable independently from the pressure and/or gasflow rate at the discharge openings of secondary channel systems inanother air bearing group.

The separate control of the pressure and/or flow rate in any of the airbearing groups may lead to difference in pressure and/or flow ratebetween different air bearing groups, and therewith in differences inthe magnitude of the local pulling and/or pushing forces exerted on theobject at any given time. Alternatively or in addition, with thisfeature it is possible to control the timing of the activation of therespective air bearing groups. For example, one air bearing group may beactivated only after another air bearing group has been activated.

Activating one air bearing group only after another air bearing grouphas been activated can be advantageous when the shape of the object onor above the member 10 has to be corrected. For example, if the objectis round, but dome-shaped instead of flat, and the embodiment of FIG. 4is used, it is advantageous to first activate the air bearing device 20*at the centre of the member 10. When the centre of the object is broughtat the desired level above the member 10 by the combined action of thepulling force and pushing forces of the air bearing device 20* at thecentre of the member 10, the air bearing devices 20 at the inner ring 14are activated. They bring a ring around the centre of the object todesired level above the member 10. The object can move freely in the x-yplane because there is no friction with any mechanical support members,which reduces the internal stresses in the object. After activating theair bearing devices in the inner ring 14, the air bearing devices 20 inthe outer ring 15 are activated. They bring an outer ring of the objectto desired level above the member 10. This way, the object is flattenedand the dome shape is reduced or even made to disappear. Afterflattening, the object may be clamped or otherwise fixed to or relativeto the member 10.

The invention may be applied in a stage system, for example in a stagesystem such as used in a lithographic apparatus. In that case, themember may be an object table or a substrate table.

So, in a further embodiment, the invention provides a stage system, forpositioning an object, which stage system comprises:

-   an object table adapted to support the object to be positioned,-   which object table is provided with a plurality of air bearing    devices,-   wherein each air bearing device comprises:    -   an air bearing body, which has a free surface,    -   a primary channel which extends through the air bearing body and        has an inlet opening in the free surface,    -   a secondary channel system which extends through the air bearing        body and which has a plurality of discharge openings in the free        surface,-   wherein the flow resistance in the secondary channel system is    higher than the flow resistance in the primary channel.

The invention allows the object, e.g. a substrate W or a patterningdevice MA, to be positioned in a direction perpendicular to a surface ofthe object table prior to clamping or otherwise fixing the object tothat surface of the object table. In some embodiments, the inventionallows to change or correct the shape of the object before prior toclamping or otherwise fixing the object the object table.

FIG. 5 shows a side view of an example of an object table 110 of stagesystem of a lithographic apparatus.

The top side of the object table 110 comprises a vacuum clamp 104 toclamp an object, e.g., a substrate W, on the object table 110. Theobject table 110 further comprises three retractable pins 1055, alsoknown as e-pins, which are movable with respect to the object tablebetween an extended position in which the pins 105 extend from theobject table 110 and a retracted position in which the pins 105 areretracted in the object table 110. The retractable pins 105 are movablein a substantially vertical direction, i.e., in a directionsubstantially perpendicular to a main plane of an object to be supportedby the pins. The retractable pins 105 are used for transfer of anobject, e.g. a substrate W, between the object table 110 and a robot orany other type of object handler. The retractable pins 105 are providedso that e.g. a gripper of a robot may be placed under the object forsupporting the object. When the robot is configured to hold the objectat the sides or top, the retractable pins 105 may be omitted. Inalternative embodiments any other type of device capable of exerting anattraction force on an object, such as electrostatic, magnetic orelectromagnetic clamps may be used.

In this embodiment a robot places an object on the pins 105 which are inthe extended position. Then the pins 105 are be moved to the retractedposition so that the object comes to rest on the support surface of theobject table 110. After an object supported by the object table 110 isfor example exposed to a patterned beam of radiation, the object isexchanged for another one. For exchange of the object it is lifted fromthe object table 110 by the retractable pins 105 which are moved fromthe retracted position to the extended position. When the retractablepins 105 are in the extended position, the object is taken over by therobot or any other type of object handler.

The vacuum clamp 104 is formed by a recessed surface 106 which issurrounded by a sealing rim 107. A suction conduit 108 is provided tocreate a low pressure in a vacuum space delimited by the recessedsurface 106, the sealing rim 107 and an object placed or to be placed onthe object table 110. The suction conduit 108 is connected to a suctionpump to draw air, or another gas present in the process environment, outof the vacuum space. The lower pressure provides a vacuum force whichdraws an object placed within a certain range above the supportingsurface towards the object table 110 in order to clamp it to the objecttable.

In the recessed surface 106 a number of burls 109 are arranged. The topends of the burls 109 provide support surfaces for an object to beplaced on the object table 110. The sealing rim 107 and the top ends ofthe burls 109 may be arranged in substantially the same plane to providea substantial flat surface for supporting an object. In an alternativeembodiment the sealing rim 107 may be arranged lower than the burls 109,as shown in FIG. 5 , or vice versa.

In the object table 110, a plurality of air bearing devices 20 isarranged. These air bearing devices are for example air bearing devicesas shown in FIGS. 2, 3 and 4 .

Each air bearing device comprises a primary channel 23 which ispreferably connected to a source 16 of sub-atmospheric pressure viachannel 12.

Each air bearing device further comprises a secondary channel system 25is formed by interconnected cavities in a porous material. The secondarychannel system is preferably connected to a source 17 of pressurised gasvia channel 13.

FIG. 6 shows an example of an air bearing device 20 as can be used inthe embodiment of FIG. 5 .

The object table 110 is provided with a plurality of air bearing devices20. Each air bearing device 20 comprises an air bearing body 21, whichhas a free surface 22. The free surface 22 is not covered by otherphysical components of the system. In use, and in particular after anoptional clamping the object on the member 10, the free surface 22 maybe covered by the object that is supported on the object table 110. Inthe example of FIGS. 5 and 6 , the free surfaces 22 of the air bearingbodies 21 extend somewhat above the surface 111 of the object table, butalternatively they can be arranged flush with the surface 111 of theobject table 110.

Each air bearing device 20 further comprises a primary channel 23 whichextends through the air bearing body 21. The primary channel 23 has aninlet opening 24 in the free surface 22.

Each air bearing device further comprises a secondary channel system 25which extends through the air bearing body 21. The secondary channelsystem 25 has a plurality of discharge openings 26 in the free surface22 of the air bearing body 21.

In the example shown in FIGS. 5 and 6 , the air bearing body 21comprises a porous material having interconnected cavities. Thesecondary channel system 25 is formed by the interconnected cavities inthe porous material. The porous material is for example a sinteredceramic material, e.g. SiSiC. Optionally, the faces of the porousmaterial through which no pressurised gas should escape are sealed.

In an alternative embodiment, not shown in the drawing, the secondarychannel system 25 in the air bearing body may be formed by channels thatare for example machined, e.g. drilled, or etched into the air bearingbody.

Preferably, the primary channel 23 is connectable to a source ofsub-atmospheric pressure 16, and the secondary channel system 25 isconnectable to a source of pressurised gas. In use, when an object isarranged on or above the surface 111, a pressurised gas can be suppliedbetween the object and the surface via the discharge openings 26 of thesecondary channel system and/or between the object and the free surfaces22. At the same time, a sub-atmospheric pressure is provided at theinlet 24 of the primary channel 23. The pressurised gas pushes theobject away from the surface 111, while the sub-atmospheric pressurepulls the object towards surface 111. The balance between the pushingforce that the pressurised gas exerts on the object and the pullingforce that the sub-atmospheric pressure exerts on the object allows toposition the object in z-direction relative to the surface 111, whichsurface 111 extends in the x-y plane.

The flow resistance in the secondary channel 25 system is higher thanthe flow resistance in the primary channel 23. This stabilises thesystem, because the higher flow resistance prevents that a force inz-direction, for example caused by tilting of the object around thex-axis or y-axis, forces the pressurised air back into the secondarychannel system 25. If pressurised gas would be forced back into thesecondary channel system, the balance between the pushing force and thepulling force would be disturbed (at least temporarily), making thesystem unstable. It even may result in that the object hits the surface111 before the balance between the pushing force and the pulling forceis re-established. This is called “hammering”, and is not desired. Inaddition, the feature that the flow resistance in the secondary channel25 system is higher than the flow resistance in the primary channel 23results in a high tilt stiffness of the system. Preferably, the flowresistance in the secondary channel 25 system is also higher than theflow resistance in the gap between the free surface 22 and the object tobe positioned.

So, the flow resistance in the secondary channel 25 system being higherthan the flow resistance in the primary channel 23 increases thestability and the reliability of the system.

In this embodiment, the object table 110 may comprise channels 12 whichallow to connect the primary channel 23 of the air bearing device 20 tothe source of sub-atmospheric pressure and further channels 13 toconnect the secondary channel system 25 of the air bearing device 20 tothe source of pressurised gas.

In the example of FIGS. 5 and 6 , in the air bearing devices 20 thedischarge openings 26 of the secondary channel system 25 surround theinlet opening 24 of the primary channel 23. This provides furtherstability to the system, because the pushing force exerted by thepressurised gas is distributed over a relatively large area, often evendistributed evenly over this area. In addition, pushing forces onopposite sides of the inlet opening 24 of the primary channel 23 maycounteract each other, which also further stabilises the system. Thisfurthermore results in a high tilt stiffness of the system. This effectis even more substantial when a porous material is used for the airbearing body, because of the large number of discharge openings 26 insuch an embodiment.

In a variant of the embodiment of FIGS. 5 and 6 , the system furthercomprises an sub-atmospheric pressure source 16 which is in fluidcommunication with the primary channel 23 of at least one air bearingdevice 20. Alternatively or in addition, in this variant, the systemfurther comprises a source 17 of pressurised gas which is in fluidcommunication with the secondary channel system 25 of at least one airbearing device 20.

Optionally, in the embodiment of FIGS. 5 and 6 , the air bearing devices20 are arranged in a rectangular grid, for example as shown in FIG. 3 ,or alternatively in a polar grid, for example as shown in FIG. 4 . Theuse of arrangements of FIG. 3 or FIG. 4 in the stage system of FIGS. 5and/or 6 , allows to change the shape of the object. For example it ispossible to correct warp, concavity, convexity, or another low orderZernike shape, or local deformation or other deviations from the flatshape of the object.

In the embodiment shown in FIGS. 5 and 6 , the air bearing bodies 21have a rectangular free surface 22, and are generally box-shaped.Alternatively, other shapes are possible, for example cylindrical airbearing bodies, e.g. with a circular or elliptical free surface.

Optionally, the embodiment of the FIGS. 5 and 6 , the plurality of airbearing devices 20 is adapted to change the shape of an object which isto be arranged on the object table 110 by controlling the pressureand/or gas flow rate at the inlet opening 24 of a primary channel 23and/or the pressure and/or gas flow rate at the discharge openings 26 ofthe secondary channel system 25 of at least one air bearing device 20.

Optionally, the control of the pressure and/or gas flow rate at theinlet 24 of a primary channel 23 and/or the pressure and/or gas flowrate at the discharge openings 26 of a secondary channel system 25 of atleast one air bearing device 20 is based on the expected and/or measuredshape of the object.

In a variant, the embodiment of the FIGS. 5 and 6 further comprises ameasurement system which is adapted to obtain shape data of an objectarranged on or above the object table 110 during activation of the airbearing devices 20. The measurement system optionally comprisescapacitive sensors, and/or inductive sensors and/or an interferometer.Optionally, one or more parts of the measurement system are arranged onor in the object table 110, so that measurement of the object can takeplace while the object is arranged at or above the object table 110.Alternatively or in addition, the measurement system may be arrangedsuch that measurement of the object takes place before the object isarranged at or above the object table 110.

In this variant, the embodiment further comprises a control device forcontrolling the pressure and/or gas flow rate at the inlet opening 24 ofa primary channel 23 and/or the pressure and/or gas flow rate at thedischarge openings 26 of a secondary channel system 25 of at least oneair bearing device 20. The control device is adapted to receive theshape data from the measurement system and to control the pressureand/or gas flow rate at the inlet opening 24 of the primary channel 23and/or the pressure and/or gas flow rate at the discharge openings 26 ofthe secondary channel system 25 of the at least one air bearing device20, based on the received shape data.

Optionally, in the embodiment of the FIGS. 5 and 6 , the plurality ofair bearing devices comprises multiple air bearing groups. Each airbearing group comprises at least one air bearing device 20.

Within an air bearing group, the pressure and/or gas flow rate at theinlets of the primary channels is in this embodiment controllableseparately from the pressure and/or gas flow rate at the inlet openingof the primary channels in another air bearing group. This allows tocontrol the local forces that are exerted on the object, which allows tochange the shape of the object. Optionally, the pressure and/or gas flowrate at the inlet opening of the primary channels is controllableindependently from the pressure and/or gas flow rate at the inletopenings of the primary channels in another air bearing group.

Alternatively or in addition, the pressure and/or the gas flow rate atthe discharge openings of the secondary channel systems in at least oneair bearing group is controllable separately from the pressure and/orgas flow rate at the discharge openings of secondary channel systems inanother air bearing group. This allows to control the local forces thatare exerted on the object, which allows to change the shape of theobject. Optionally, the pressure and/or the gas flow rate at thedischarge openings of the secondary channel systems in at least one airbearing group is controllable independently from the pressure and/or gasflow rate at the discharge openings of secondary channel systems inanother air bearing group.

The separate control of the pressure and/or flow rate in any of the airbearing groups may lead to differences in pressure and/or flow ratebetween different air bearing groups, and therewith in differences inthe magnitude of the local pulling and/or pushing forces exerted on theobject at any given time. Alternatively or in addition, with thisfeature it is possible to control the timing of the activation of therespective air bearing groups. For example, one air bearing group may beactivated only after another air bearing group has been activated.

Activating one air bearing group only after another air bearing grouphas been activated can be advantageous when the shape of the object onor above the member 10 has to be corrected. For example, if the objectis round, but dome-shaped instead of flat, and the configuration of FIG.4 is used in the embodiment of FIGS. 5 and 6 , it is advantageous tofirst activate the air bearing device 20* at the centre of the objecttable 110. When the centre of the object is brought at the desired levelabove the object table 110 by the combined action of the pulling forceand pushing forces of the air bearing device 20* at the centre of theobject table 110, the air bearing devices 20 at the inner ring 14 areactivated. They bring a ring around the centre of the object to desiredlevel above the object table 110. The object can move freely in the x-yplane because there is no friction with any mechanical support members,which reduces the internal stresses in the object. After activating theair bearing devices in the inner ring 14, the air bearing devices 20 inthe outer ring 15 are activated. They bring an outer ring of the objectto desired level above the object table 110. This way, the object isflattened and the dome shape is reduced or even made to disappear. Afterflattening, the object may be clamped or otherwise fixed to or relativeto the object table 110.

FIG. 7 shows a variant of a stage system according to the invention.

In the variant of the object table 110 comprises a porous zone 110 a.Optionally, also a non-porous zone 110 b is present. At least one airbearing body 21 forms part of the porous zone 110 a of the object table110.

In the embodiment shown in FIG. 7 , impermeable elements 27 have beenprovided in order to avoid leaking of pressurised gas when the porousair bearing bodies 21 have been connected to a source of pressurisedgas.

For example, the object table 110 is in this variant made from asintered ceramic, e.g. from SiSiC. After sintering, the entire objecttable 110 is porous. Then, a sealant substance is allowed to fill theinterconnected cavities, e.g. by arranging the lower part from theobject table in a bath of liquid sealant, which enters theinterconnected cavities due to capillary action. This process is howeverstopped before the entire object table is saturated. Only a lower partof the object table 110 is filled with the sealant. This lower partforms the non-porous zone 110 b. The impermeable elements 27 may beformed by locally injecting sealant.

In the embodiments described, the air bearing device 20 is arranged todischarge air via the discharge opening 26. The discharged air may havethe same composition as the ambient air, or may have a differentcomposition than the ambient air. For example, the humidity of thedischarged air may be different, for example lower, than that of theambient air. The discharged air may comprise any type of suitable gas,such as nitrogen or carbon dioxide. It is clear to the skilled personthat the term ‘air bearing’ may be interpreted as a more general term‘gas bearing’.

In an embodiment, the invention further provides a method of positioningan object, which method comprises the following steps:

-   -   arranging an object on or above the object table of a stage        system according to the invention,    -   making pressurized gas flow out of the discharge openings of the        secondary channel systems of at least one air bearing device of        the stage system according to the invention while simultaneously        applying a sub-atmospheric pressure to the inlet of the primary        channel of at least one air bearing device of the stage system        according to the invention, thereby keeping the object in a        position spaced apart from the object table in a direction        perpendicular to the object table.

In a possible embodiment of the stage system according to the invention,the plurality of air bearing devices comprises multiple air bearinggroups each comprising at least one air bearing device, wherein thepressure and/or gas flow rate at the inlet opening of the primarychannel or at the inlet openings of the primary channels, respectively,in at least one air bearing group of the stage system is controllableseparately from the pressure and/or gas flow rate at the inlet openingof the primary channel or primary channels, respectively, in another airbearing group in the stage system.

In a possible embodiment of the method according to the invention, suchan embodiment of the stage system is used. In this embodiment of themethod according to the invention, the pressure and/or gas flow rate atthe inlet of the primary channel or at the inlets of the primarychannels, respectively, in at least one air bearing group of the stagesystem is controlled separately from the pressure and/or gas flow rateat the inlet of the primary channel or primary channels, respectively,in another air bearing group of the stage system so as to control theshape of the object.

In a possible embodiment of the stage system according to the invention,the plurality of air bearing devices comprises multiple air bearinggroups each comprising at least one air bearing device, wherein thepressure and/or the gas flow rate at the discharge openings of thesecondary channel system or secondary channel systems, respectively, inat least one air bearing group of the stage system is controllableseparately from the pressure and/or gas flow rate at the dischargeopenings of the secondary channel system or secondary channel systems,respectively, in another air bearing group of the stage system.

In a possible embodiment of the method according to the invention, suchan embodiment of the stage system is used. In this embodiment of themethod according to the invention, the pressure and/or the gas flow rateof at the discharge openings of the secondary channel system orsecondary channel systems, respectively, in at least one air bearinggroup of the stage system, is controlled separately from the pressureand/or gas flow rate at the discharge of the secondary channel system orsecondary channel systems, respectively, in another air bearing group ofthe stage system so as to control the shape of the object.

In a possible embodiment of the method according to the invention, themethod further comprises changing the shape of the object which is to bearranged on the object table by controlling the pressure and/or gas flowrate at the inlet opening of a primary channel and/or the pressureand/or gas flow rate at the discharge openings of the secondary channelsystem of at least one air bearing device.

Optionally, this controlling of the pressure and/or gas flow rate at theinlet opening of a primary channel and/or of the pressure and/or gasflow rate at the discharge openings of the secondary channel system isbased on an expected and/or measured shape of the object.

In another embodiment of the invention, there is provided a system forpositioning an object, which system comprises:

-   -   a member, which member is provided with a plurality of air        bearing devices,    -   wherein each air bearing device comprises:    -   an air bearing body, which has a free surface,    -   a primary channel which extends through the air bearing body and        has an inlet opening in the free surface,    -   a secondary channel system which extends through the air bearing        body and which has a plurality of discharge openings in the free        surface,    -   wherein the flow resistance in the secondary channel system is        higher than the flow resistance in the primary channel,    -   wherein the plurality of air bearing devices is adapted to        change the shape of an object which is to be arranged on the        object table by controlling the pressure and/or gas flow rate at        the inlet opening of a primary channel and/or the pressure        and/or gas flow rate at the discharge openings of the secondary        channel system of at least one air bearing device.

Optionally, in this embodiment, the control of the pressure and/or gasflow rate at the inlet of a primary channel and/or the pressure and/orgas flow rate at the discharge openings of a secondary channel system ofat least one air bearing device is based on the expected and/or measuredshape of the object.

Optionally, in this embodiment, the system further comprises ameasurement system which is adapted to obtain shape data of an objectarranged on the object table during activation of the air bearingdevices, and further comprises a control device for controlling thepressure and/or gas flow rate at the inlet of a primary channel and/orthe pressure and/or gas flow rate at the discharge openings of asecondary channel system of at least one air bearing device. The controldevice is adapted to receive the shape data from the measurement systemand to control the pressure and/or gas flow rate at the inlet opening ofthe primary channel and/or the pressure and/or gas flow rate at thedischarge opening of the secondary channel system of the at least oneair bearing device based on the received shape data.

In another embodiment of the invention, there is provided a method forpositioning an object, which method comprises the following steps:

-   -   arranging an object on or above the surface of a member of a        system, which member is provided with a plurality of air bearing        devices,    -   wherein each air bearing device comprises:    -   an air bearing body, which has a free surface,    -   a primary channel which extends through the air bearing body and        has an inlet opening in the free surface,    -   a secondary channel system which extends through the air bearing        body and which has a plurality of discharge openings in the free        surface,    -   wherein the flow resistance in the secondary channel system is        higher than the flow resistance in the primary channel,    -   making pressurized gas flow out of the discharge openings of the        secondary channel systems of at least one air bearing device        while simultaneously applying a sub-atmospheric pressure to the        inlet of the primary channel of at least one air bearing device,        thereby keeping the object in a position spaced apart from the        member in a direction perpendicular to the surface of the        member,    -   wherein the method further comprises changing the shape of the        object which is to be arranged on the object table by        controlling the pressure and/or gas flow rate at the inlet        opening of a primary channel and/or the pressure and/or gas flow        rate at the discharge openings of the secondary channel system        of at least one air bearing device.

Optionally, this controlling of the pressure and/or gas flow rate at theinlet opening of a primary channel and/or of the pressure and/or gasflow rate at the discharge openings of the secondary channel system isbased on an expected and/or measured shape of the object.

In an embodiment, there is provided a stage system for positioning anobject, the stage system comprising an object table adapted to supportthe object to be positioned, the object table having a plurality of airbearing devices, wherein each air bearing device comprises: an airbearing body having a free surface, a primary channel which extendsthrough the air bearing body and has an inlet opening in the freesurface, and a secondary channel system which extends through the airbearing body and which has a discharge opening in the free surface,wherein the flow resistance in the secondary channel system is higherthan the flow resistance in the primary channel.

In an embodiment, the air bearing body comprises a porous materialcomprising interconnected cavities, and the secondary channel system isformed by the interconnected cavities. In an embodiment, the stagesystem comprises a plurality of discharge openings, wherein theplurality of discharge openings surround the inlet opening of theprimary channel. In an embodiment, the plurality of air bearing devicescomprises multiple air bearing groups each group comprising at least oneair bearing device, wherein the pressure and/or gas flow rate at the oneor more inlet openings of the one or more primary channels in at leastone air bearing group is controllable separately from the pressureand/or gas flow rate at the one or more inlet openings of the one ofmore primary channels in another air bearing group. In an embodiment,the plurality of air bearing devices comprises multiple air bearinggroups each group comprising at least one air bearing device, whereinthe pressure and/or the gas flow rate at the one or more dischargeopenings of the one or more secondary channel systems in at least oneair bearing group is controllable separately from the pressure and/orgas flow rate at the one or more discharge openings of the one or moresecondary channel systems in another air bearing group. In anembodiment, the plurality of air bearing devices is adapted to change ashape of an object which is to be arranged on the object table bycontrolling the pressure and/or gas flow rate at the inlet opening of aprimary channel and/or the pressure and/or gas flow rate at thedischarge opening of the secondary channel system of at least one airbearing device. In an embodiment, the control of the pressure and/or gasflow rate at the inlet of a primary channel and/or the pressure and/orgas flow rate at the discharge opening of a secondary channel system ofat least one air bearing device is based on an expected and/or measuredshape of the object. In an embodiment, the stage system furthercomprises a measurement system which is adapted to obtain shape data ofthe object when arranged on the object table during activation of theair bearing devices, and a control device for controlling the pressureand/or gas flow rate at the inlet of a primary channel and/or thepressure and/or gas flow rate at the discharge opening of a secondarychannel system of at least one air bearing device, wherein the controldevice is adapted to receive the shape data from the measurement systemand to control the pressure and/or gas flow rate at the inlet opening ofthe primary channel and/or the pressure and/or gas flow rate at thedischarge opening of the secondary channel system of the at least oneair bearing device based on the received shape data. In an embodiment,the object table comprises a porous zone, and at least one air bearingbody forms part of the porous zone of the object table.

In an embodiment, there is provided a lithographic apparatus arranged totransfer a pattern from a patterning device onto a substrate, whereinthe lithographic apparatus comprises a stage system as described herein.

In an embodiment, there is provided a lithographic apparatus comprising:an illumination system configured to condition a radiation beam; asupport constructed to support a patterning device, the patterningdevice being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam; a substrate tableconstructed to hold a substrate; and a projection system configured toproject the patterned radiation beam onto a target portion of thesubstrate, wherein the substrate table has a plurality of air bearingdevices, each air bearing device comprising: an air bearing body havinga free surface, a primary channel which extends through the air bearingbody and has an inlet opening in the free surface, and a secondarychannel system which extends through the air bearing body and which hasa discharge opening in the free surface, wherein the flow resistance inthe secondary channel system is higher than the flow resistance in theprimary channel.

In an embodiment, there is provided a device manufacturing methodcomprising transferring a pattern from a patterning device onto asubstrate, wherein use is made of a stage system as described herein.

In an embodiment, there is provided a method for positioning an object,the method comprising: arranging an object on or above the object tableof a stage system as described herein; and making pressurized gas flowout of the discharge opening of the secondary channel system of at leastone air bearing device of the stage system while simultaneously applyinga sub-atmospheric pressure to the inlet of the primary channel of atleast one air bearing device of the stage system thereby keeping theobject in a position spaced apart from the object table in a directionperpendicular to the object table.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

What is claimed is:
 1. A stage system for positioning an object, thestage system comprising: a member having a plurality of gas bearingdevices arranged to support the object; and a control device, whereineach gas bearing device comprises a free surface having multipledischarge openings in the free surface and an inlet opening in the freesurface, the multiple discharge openings are formed in a porous materialand the inlet opening is surrounded with the multiple dischargeopenings, wherein the free surface is arranged to support and contactthe object at least during a materials processing of the object, whereinthe discharge openings are arranged to provide pressurized gas and theinlet opening is arranged to provide a sub-atmospheric pressure, andwherein the control device is arranged to change a shape of the objectby controlling a pressure and/or a gas flow rate at the dischargeopenings of at least one of the gas bearing devices and/or bycontrolling a pressure and/or a gas flow rate at the inlet opening of atleast one of the gas bearing devices.
 2. The stage system of claim 1,wherein the gas bearing devices are circular.
 3. The stage system ofclaim 1, further comprising a measurement system configured to obtainshape data of the object arranged on or above the member duringactivation of the gas bearing devices.
 4. The stage system of claim 3,wherein at least a part of the measurement system is arranged on themember.
 5. The stage system of claim 3, wherein the measurement systemcomprises a capacitive sensor.
 6. The stage system of claim 3, whereinthe control device is configured to receive the shape data from themeasurement system to control a pressure and/or a gas flow in thedischarge openings of at least one of the gas bearing devices.
 7. Thestage system of claim 1, wherein the plurality of gas bearing devicescomprises multiple gas bearing groups, and wherein the control device isconfigured to control a pressure and/or gas flow rate at the dischargeopenings in at least one gas bearing group separately from the pressureand/or gas flow rate at the discharge openings in at least one other gasbearing group.
 8. The stage system of claim 7, wherein the controldevice is configured to control a timing of an activation of respectivegas bearing groups.
 9. The stage system of claim 1, wherein the controldevice is configured to change the shape of the object by controlling apressure and/or gas flow rate at the inlet opening of at least one ofthe gas bearing devices.
 10. A lithographic apparatus arranged totransfer a pattern from a patterning device onto a substrate, whereinthe lithographic apparatus comprises the stage system according toclaim
 1. 11. A lithographic apparatus comprising: a support constructedto support a patterning device, the patterning device being capable ofimparting a radiation beam with a pattern in its cross-section to form apatterned radiation beam; a substrate table constructed to hold asubstrate; and a projection system configured to project the patternedradiation beam onto a target portion of the substrate, wherein thesubstrate table comprises the stage system of claim 1 and the objectcomprises the substrate.
 12. The stage system of claim 1, wherein a flowresistance to a flow of gas into the porous medium is larger than a flowresistance to a flow of gas into the inlet opening.
 13. The stage systemof claim 12, wherein flow resistance is a characteristic respectively ofthe discharge openings structure itself and of the inlet openingstructure itself.
 14. The stage system of claim 1, wherein a materialsprocessing of the object comprises exposing the object toelectromagnetic radiation.
 15. A method comprising: supporting an objecton a stage, the stage comprising a member having a plurality of gasbearing devices arranged to support the object, wherein each gas bearingdevice comprises a free surface having multiple discharge openings inthe free surface and having an inlet opening in the free surface,wherein the multiple discharge openings are formed in a porous material,wherein the inlet opening is surrounded with the multiple dischargeopenings, wherein the free surface supports and contacts the object atleast during a materials processing of the object, and wherein thedischarge openings are arranged to provide pressurized gas and the inletopening is arranged to provide a sub-atmospheric pressure; and changinga shape of the object by controlling a pressure and/or a gas flow rateat the discharge openings of at least one of the gas bearing devicesand/or by controlling a pressure and/or a gas flow rate at the inletopening of at least one of the gas bearing devices.
 16. The method ofclaim 15, further comprising using a measurement system to obtain shapedata of the object arranged on or above the member during activation ofthe gas bearing devices, wherein the controlling is based on the shapedata.
 17. The method of claim 16, wherein at least a part of themeasurement system is arranged on the member.
 18. The method of claim15, wherein the plurality of gas bearing devices comprises multiple gasbearing groups, and wherein the controlling comprises controlling apressure and/or gas flow rate at the discharge openings in at least onegas bearing group separately from the pressure and/or gas flow rate atthe discharge openings in at least one other gas bearing group.
 19. Themethod of claim 18, comprising controlling a timing of an activation ofrespective gas bearing groups.
 20. The method of claim 15, wherein thecontrolling comprises controlling a pressure and/or gas flow rate at theinlet opening of at least one of the gas bearing devices.
 21. The methodof claim 15, further comprising transferring a pattern from a patterningdevice onto a substrate as the object.
 22. A stage system forpositioning an object, the stage system comprising: a member having aplurality of gas bearing devices arranged to support the object; and acontrol device, wherein each gas bearing device comprises a free surfacehaving a porous medium in the free surface and having an inlet openingin the free surface connected to a channel formed in the porous medium,wherein the porous medium is arranged to provide pressurized gas to theobject and the inlet opening is arranged to provide a sub-atmosphericpressure to the object, wherein the porous medium is exposed to an openinterior of the channel on at least part of an interior surface of thechannel and has a sealing material contacted on or injected in at leasta surface region of the porous medium at a location away from theinterior surface of the channel and contacted on or injected in at leastpart of the interior surface of the channel, and wherein the controldevice is arranged to change a shape of the object by controlling apressure and/or a gas flow rate of the pressurized gas of at least oneof the gas bearing devices and/or by controlling a pressure and/or a gasflow rate at the inlet opening of at least one of the gas bearingdevices.
 23. The stage system of claim 22, wherein the pressurized gasproviding porous medium surrounds the inlet opening.